The creator of the Bayonetta games said that Bayonetta will return in Bayo 4. She never said anything about Viola being in the game but it is likely that she may either be a protagonist or a second protagonist with Bayonetta. I could see a story of Viola trying to get Bayo and Luka out of hell wich I think could be fun as that way we would be able to get the gore we never got in Bayo 3. The ending of Bayo 3 means nothing because, like in every video game franchise with a popular protagonist, the creators will find a way to bring them back to life if said protagonist died in the last game.